The present invention relates to a combined cycle power plant and especially relates to a method and system for controlling water level of a boiler drum in the combined cycle power plant.
The combined cycle power plant generally includes a steam turbine combined with at least one set of a gas turbine and heat recovery steam generator, in which exhaust gas from the gas turbine is led to the heat recovery steam generator, and the steam generated there is utilized for the steam turbine. In the above combined cycle power plant, there are several kinds of water reservoirs such as boiler (steam generator) drum, deaerator reservoir and condenser reservoir. In these reservoirs, the boiler drum functions to provide a stable water supply to the steam generator and also to accumulate heat energy for compensating the changes in the amount of steam generated or consumed in the power plant. Generally, the water level of the boiler drum is preferably controlled to be at a constant water level by adding water into or blowing water out of the boiler drum. Especially, inside the boiler drum there is provided a water vapor separator so that wet steam may not be introduced into the steam turbine. Therefore, it is necessary to control the water level in the boiler drum such that the water level may not exceed the predetermined water level above which the water vapor separator is located.
However, in a starting operation of the combined cycle power plant, the water level in the boiler drum rises abruptly. This is because, when the water in the steam generator reacts with the high temperatured gas turbine exhaust gases, the temperature of the water in the steam generator rises rapidly and evaporation starts suddenly. In this state, vapor phase and liquid phase coexist in the steam generator in an equilibrium state, therefore the average specific volume (m.sup.3 /kg) of the water swells and the observed volume (m.sup.3) of the water increases, which causes an abrupt water level rise in the boiler drum. This water level rise may exceed the highest water level allowed and thereby create a problem of safety. Accordingly, when this water level rise happens, it is necessary to blow out the excessive water out of the drum. However, the water level rise is so rapid and temporary that skilled controls are necessary to blow out the excessive water, otherwise serious safety problems are encountered especially in case where the combined cycle power plant is periodically required to start up and shut down operation every day. Also, heat losses by blowing out the excessive water are large and not negligible.
An ASME PUBLICATION, 75-PWR-23 entitled "Automation of Multi-Generator Combined Cycle Plants" recognizes the problem and suggests that magnitude of the drum level increase can be lowered by changing the steam line valve opening sequence because the "swelling phenomenon" is caused by steam pressure decrease. However, this sequence change brings extremely high pressure in the steam generator which may cause a dangerous situation and also brings delay in start up operation of the steam turbine because pressurized steam is kept in the boiler drum until swelling phenomenon is over instead of supplying into the steam turbine.
An object of the present invention is to provide an improved method and system for controlling water level of the boiler drum of a combined cycle power plant, which is capable of providing stable water level controls at the start up operation.
Another object of the present invention is to provide an improved method and system for controlling water level of the boiler drum so as to reduce heat losses at the start up operation.
According to the present invention, a method and system for controlling boiler drum water level for a combined cycle power plant is provided in which the water level of the boiler drum is advantageously controlled to be within a range defined by the highest water level and normal water level of the boiler drum, so that unstable water level rise and heat energy losses by a water level swelling phenomenon can be eliminated.
According to a feature of the present invention, the normal water control level is lowered below the normal water level in advance of the start up operation of the combined cycle power plant, so that the highest water level by the swelling phenomenon can not exceed the predetermined highest water level. The normal water control level is preferably changed in accordance with the gas turbine load change or heat energy input to the boiler.
According to another feature of the present invention, the gas turbine load is prevented from increasing for a certain period of time in which the swelling phenomenon is expected to occur, thereby the water level rise by the swelling phenomenon is decreased. The gas turbine load is preferably maintained for a certain period at a constant level.
According to still another feature of the present invention, water levels of a condenser reservoir and deaerator reservoir, wherein steam exhausted from the steam turbine is condensed and recycled to the boiler drum through the deaerator reservoir, are respectively controlled in a preferred control level, whereby water level rise in the boiler drum is eliminated. Preferably, the water level of the condenser reservoir is controlled to be constant and the water level of the deaerator reservoir is controlled in accordance with the amount of steam generated in the steam generator.
According to a still further feature of the present invention, excessive water in the boiler drum is preferably introduced into the deaerator reservoir, so that abrupt water level rise in the boiler drum at the start up operation is advantageously eliminated.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration, several embodiments in accordance with the present invention .